Not Applicable
The present invention relates generally to electromechanical valve drive systems, and more specifically to an electromechanical valve drive system incorporating a nonlinear mechanical transformer.
Traditional internal combustion (IC) engines are well known. In an IC engine, a camshaft (also referred to as simply a cam) acts on the valve stems of valves to open and close the valves. The timing of the valves"" openings and closings is controlled by the cam design and is fixed relative to piston position since the cam is physically coupled to and driven by the crankshaft. Due to this fixed relationship between the camshaft and crankshaft, the valve timing in IC engines is designed optimally at one speed and load, usually, at high speed and wide-open throttle conditions.
Alternates to IC engines are also known. One such alternative is a variable valve actuation (VVA) system in which significant improvements in fuel efficiency, engine performance, emission, and idle quality has been achieved. One of the most advanced VVA systems demonstrated to date is the BPVD (bi-positional electromechanical valve drive), which can offer cylinder deactivation, as well as duration and phase control functions, without a camshaft. Such a BPVD VVA assembly comprises a valve or valves, one or more springs, and an electromechanical actuator. In a particular BPVD, two solenoids are used as the electromechanical actuator. The spring (or system of springs) is disposed such that the zero-force position for the springs is at the midpoint of the valve stroke. The acceleration curve in BPVD systems has a relatively large (theoretically infinite) time rate of change of acceleration (referred to as xe2x80x9cjerkxe2x80x9d) at both ends of the stroke which provides a harsh landing of the valve at the end of the stroke. This is one of the reasons why the idealized prior BPVD must be modified or intensively controlled to achieve a soft landing.
Even the best prior art EMVD""s are very noisy due at least in part to the large jerk at both ends of the stroke. In order to reduce the large jerk associated with the prior EMVD and to reject external disturbances, active feedback control is implemented. However, in prior EMVDs with active feedback control, there are two critical problems. The solenoid actuators (which are a member of the class of normal-force electromagnetic actuators, in which the force acts normal to the air gap surface) have the property that the force of a given actuator is unidirectional. Thus to provide a bi-directional force capability, two oppositely directed actuators are required. Solenoid actuators also have the property that the force coefficient (force per unit current) falls off rapidly as air gap increases. As the valve approaches its intended resting place at the end of a stroke, the near actuator can easily provide a large force to draw the valve to its resting place. It is difficult not to apply too much force, contributing to a hard landing. If at any point in the transition too much force in the direction of motion has been applied, the valve will approach the end of stroke too fast, and will collide forcefully with the stop at the end of the stroke. The actuator which is capable of supplying force in the direction to slow the valve near the end of stroke must act with a large air gap. That actuator will have a small force coefficient and may be unable to apply enough retarding force, even with high current. Once the valve has come to rest, the normal force actuator which holds it at rest works with a small air gap. It can therefore hold the valve at rest with a low current.
For ease of control, a shear force actuator is much to be preferred. These actuators are bidirectional, so the same actuator can provide force in either direction. They are commonly produced with a force coefficient which does not vary as a function of the position of the valve. This linearizes and simplifies the control problem. But simple substitution of a shear force actuator for the solenoids in existing BPVD""s is not the answer. The holding current to maintain the valve at both ends of the stroke is undesirably high and the concomitant power loss is high as well. Additionally, the driving current is too large to be acceptable in practice.
It would, therefore, be desirable to provide an EMVD control system having a relatively low holding current and a relatively low driving current. It would be further desirable to provide an EMVD having a relatively low holding current and a relatively low driving current while also having smooth acceleration, soft valve landing, and reduced power consumption characteristics.
In accordance with the present invention, a valve drive system includes a nonlinear mechanical transformer having a motor coupled thereto. In accordance with the present invention, a valve drive system includes a nonlinear mechanical transformer having a first end coupled to a portion of the system and having a second end adapted to couple to a valve. The system further includes a motor which can be electrically controlled to drive the nonlinear mechanical transformer at different speeds independently of the engine cycle. This allows the drive system to provide fully variable valve actuation functions. Accordingly, the valve drive system of the present invention corresponds to an electromechanical valve drive (EMVD) variable valve actuation (VVA) system. Since the motor drives a nonlinear mechanical transformer, a valve drive system having a relatively low holding current and a relatively low drive current is provided. The present invention thus provides reduced holding current and driving current of an EMVD in an effective and practical manner. The present invention achieves the reduced holding current and driving current by incorporating a nonlinear mechanical transformer as part of the EMVD system. The nonlinear mechanical transformer is designed for the spring and the inertia in the EMVD to have desirable nonlinear characteristics.
In one embodiment, a spring or a system of springs is disposed about the nonlinear mechanical transformer. The nonlinear mechanical transformer is designed for the spring and the inertia in the EMVD to the value with desirable characteristics. The nonlinear characteristics of a nonlinear mechanical transformer can be implemented in various ways. Additional embodiments include an inherently nonlinear spring. The nonlinear spring may be in the form of a disk spring. The concept of using a nonlinear mechanical transformer can be applied not only to EMVD""s but also to general reciprocating and bi-positional servomechanical systems, where smooth acceleration, soft landing, and small power consumption are desired.